A well known adage states "an ounce of prevention is worth a pound of cure." Despite the wisdom offered by this adage, most implantable stimulator devices available today, e.g., pacemakers or defibrillators, are "reactive" devices. Only pharmacologic therapy (with its accompanying undesirable side effects) has made any serious attempt at preventative therapy. Implantable devices, on the other hand, are predominantly reactive. That is, such devices are designed to sense certain conditions perceived as undesirable or dangerous to a particular patient, such as a tachyarrhythmia, bradycardia, or fibrillation, and then respond to such sensed undesirable condition by automatically issuing one or more prescribed stimulation or defibrillation pulses, at prescribed rates and energy levels, in an effort to quickly terminate or stop the sensed undesirable condition. U.S. Pat. Nos. 4,548,209; 4,693,253; 4,788,980; 5,103,822; 5,188,105; for example, are illustrative of the numerous patents that exist which disclose reactive-type pacemakers that (1) sense a tachyarrhythmia and (2) react to the sensed tachyarrhythmia in an effort to quickly terminate it.
For many patients, i.e., those particularly prone or susceptible to experiencing the undesirable or potentially dangerous conditions mentioned above--tachyarrhythmia, bradycardia, or fibrillation--, it would be much more desirable to prevent the undesirable condition rather than to simply treat it (by trying to stop it) once it has occurred. Unfortunately, the only types of preventative management that have been regularly practiced in the art are fraught with difficulties and/or undesirable side effects. Such prior preventative management techniques include: pharmacologic therapy, with a high incidence of side effects (some of which are life threatening); or physically interrupting a critical conduction pathway, which first requires a complex electrophysiologic study followed by either catheter ablation or an open heart surgical operation. On occasion, pacing at a relatively rapid rate (overdrive suppression) has been effective as a preventative tool, but this unfortunately obligates the patient to being paced rapidly at times when such rapid pacing may not be required. There is thus a need in the art to identify particular pacing or stimulation strategies that, when used, prevent the onset of, or at least minimize the likelihood of occurrence of, a tachyarrhythmia or other perceived undesirable condition, and which apply such stimulation strategies only when needed and only for so long as needed.
One prior art implantable defibrillator device, disclosed in U.S. Pat. No. 5,425,749, provides a single preemptive charge to a patient's heart within a few seconds after having either detected or predicted the onset of an arrhythmia, instead of waiting up to 25 seconds to confirm an arrhythmia and then deliver a large shock. If the single preemptive charge is ineffective, then conventional detection/confirmation of the arrhythmia continues along with the more time consuming charging of a large output capacitor, and eventual delivery of the large shock. If the single preemptive charge is effective, however, then the large capacitor is not charged. In this way, the preemptive shock is applied sooner, e.g., within 2-3 seconds of when the arrhythmia is first detected or predicted.
The '749 patent teaches that the detection of an arrhythmia may be made by detecting a burst of "n" high rate heartbeats of approximately 200 beats per minute, or by predicting or anticipating the occurrence of fibrillation as described in Droll et al., "Slope Filtered Pointwise Correlation Dimension Algorithm and Evaluation with Prefibrillation Heart Rate Data," Journal of Electrocardiology, Vol. 24, Supplement, pp. 97-101. Other techniques mentioned in the '749 patent for detecting or predicting the onset of fibrillation include techniques based on waveform morphology, rate acceleration, or rate stability, although no express teachings are provided for how waveform morphology, rate acceleration, or rate stability could be used for this purpose.
Despite the teaching of the '749 patent that a single preemptive shock could be delivered by an implantable defibrillator device if an arrhythmia is detected or predicted, there remains a critical need in the art to improve upon this technique, and in particular to provide a much lower power and less traumatic pacing regime aimed at preventing an arrhythmia from occurring, including improved techniques for recognizing when and if an arrhythmia is soon likely to occur.
Coupled with the above need (of identifying appropriate preemptive tachyarrhythmia pacing or stimulation strategies) is the need to know when such pacing or stimulation strategies can or should be used to best benefit a given patient. In other words, there is a need to ascertain, for any given patient, whether such preemptive tachyarrhythmia pacing strategies should be employed continuously, or whether such can or should be used selectively, e.g., only when one or more specified parameters, monitored with one or more appropriate sensors, suggest that the onset of a tachyarrhythmia is imminent.
Additional background material relating to the causes of pathologic tachyarrhythmias, and the teachings of the prior art relative to treating such tachyarrhythmias is found in Appendix A, below.